Cohortstudies

A cohort study is an observational research design in which a defined group of individuals sharing a common characteristic or exposure is followed over time to observe the development of outcomes of interest. Cohort studies are typically used to assess associations between exposures and diseases, estimate incidence, and derive measures of risk. They can be prospective, in which exposure status is recorded at the start and participants are followed forward in time, or retrospective, in which existing records are used to reconstruct past exposure and outcomes.

In a prospective cohort, researchers enroll participants without the outcome and classify them by exposure status, then monitor for disease occurrence. In a retrospective cohort, the study uses historical data to define the cohort and determine exposure and outcome by the end of the study period, effectively reconstructing the timeline.

Key analysis focuses on incidence rates and relative measures of association such as relative risk (risk ratio) or hazard ratios from time-to-event analyses. Cohort studies can examine multiple outcomes from a single exposure and several exposures for a single outcome. They often rely on questionnaires, medical records, or linked registries for data on exposure and disease status.

Strengths include clear temporality, the ability to study multiple outcomes, and direct measurement of incidence. They are less prone to recall bias for exposures defined at baseline but can be affected by confounding and loss to follow-up. Limitations include potential for selection bias, large sample size and long duration, cost, and misclassification of exposure or outcome.

Examples: Framingham Heart Study, Nurses' Health Study; occupational cohort studies.